Production of helium from a gas stream containing hydrogen

ABSTRACT

The invention relates to a method for producing helium from a source gas stream ( 1 ) including at least helium, methane, nitrogen and hydrogen, comprising at least the following consecutive steps: step a): injecting said source gas stream ( 1 ) into at least one compressor ( 3 ); step b): eliminating the hydrogen and the methane by reacting the stream ( 4 ) obtained from step a) with oxygen; step c): eliminating at least the impurities from step b) by temperature swing adsorption (TSA); step d): partially condensing the stream ( 8 ) obtained from step c) in order to produce a stream ( 10 ) of liquid nitrogen and a gas stream ( 11 ) comprising mostly helium; step e): purifying the gas stream ( 11 ) obtained from step d) in order to increase the helium content by pressure swing adsorption (PSA) by eliminating the nitrogen and the impurities contained in the gas stream ( 11 ) obtained from step d).

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a 371 of International PCT ApplicationPCT/FR2015/052633, filed Oct. 1, 2015, which claims priority to FrenchPatent Application No. 1553906, filed Apr. 30, 2015, the entire contentsof which are incorporated herein by reference.

BACKGROUND

The present invention relates to a process for producing helium from asource gas stream comprising at least helium, methane, nitrogen andhydrogen.

Helium is obtained commercially virtually exclusively from a mixture ofvolatile components of natural gas, this mixture comprising, along withhelium, typically methane and nitrogen and traces of hydrogen, argon andother noble gases. In the course of the production of mineral oil,helium is provided as a component of the gas which accompanies themineral oil, or in the context of the production of natural gas. It istheoretically possible to obtain helium from the atmosphere, but this isnot economical on account of the low concentrations (typicalconcentration of helium in air of about 5.2 ppmv).

In order to avoid undesirable freezing during a process of liquefactionof helium, the concentration of the impurities in the helium stream tobe liquefied must not exceed a value of 1000 ppm by volume, preferably10 ppmv.

For this reason, the helium liquefaction process is connected downstreamof a helium purification process. This is generally composed of acombination of cryogenic processes, generally based on partialcondensation, and of adsorption processes, regeneration in the lattercase being possible by means of varying the temperature and/or thepressure.

In many cases, it is advantageous to perform a helium purificationprocess such that, in addition to the purified helium, nitrogen ofrequired purity—in which the sum of the impurities is less than 1% byvolume—may be obtained. In general, only a portion, typically from 5% to70%, preferably from 10% to 50%, of the nitrogen present in the mixtureto be purified is brought to the desired purity.

The remaining nitrogen is released to the atmosphere at the same time asmethane in low-pressure gas form, either directly or after an oxidationstep, preferably performed in a torch or an incinerator.

A known example of a prior art process for obtaining a fraction of purehelium from a starting fraction comprising at least helium, methane andnitrogen is described in patent application AU 2013/200 075.

This process for obtaining a fraction of pure helium from a startingfraction comprising at least helium, methane and nitrogen comprises thefollowing successive steps:

a) the starting fraction is subjected to a removal of methane andnitrogen,

b) the fraction obtained from a) which is composed essentially of heliumand nitrogen is compressed,

c) the compressed fraction is subjected to a removal of nitrogen, and

d) the helium-rich fraction obtained in step c) is subjected topurification by adsorption to produce a fraction.

Early removal of the methane fraction contained in the initial gasstream to be treated imposes the use of two necessary independentcryogenic steps, and the investment and running costs are thussubstantial.

Moreover, part of the nitrogen contained in the initial gas stream to betreated is lost with the ethane removed in the first step. Now, therecycling of nitrogen for other applications is a key element on anindustrial scale, since nitrogen, in particular liquid nitrogen, ishighly economically upgradable.

In addition, this process does not make it possible to treat gas streamscontaining a high content of hydrogen, typically more than 6% by volumeof hydrogen.

Another type of helium purification process known from the prior art isillustrated by FIG. 1.

A gas stream 1′ comprising nitrogen, methane, helium and hydrogen, forexample originating from the outlet of a nitrogen rejection unit (NRU)15′ following the treatment of a natural gas stream to remove thenitrogen from this natural gas, is introduced into a compressor 2′. Oncethis gas has been compressed, it is introduced into ahelium-concentrating device 3′.

At the outlet of this device 3′, the hydrogen contained in the gasstream is removed by means of a system 4′ in which hydrogen and oxygenreact.

On conclusion of this step, the gas stream is then purified by means 5′of a pressure swing adsorption (PSA) process. A gas stream 6′,originating from the PSA 6′, predominantly containing helium is thenliquefied in a helium liquefaction device 7′. The liquefied helium issent to a helium storage system 8′. Said storage system 8′ is cooledwith liquid nitrogen 9′ obtained from a liquid nitrogen storage device10′ fed by an air-separating unit 11′.

Moreover, the liquid nitrogen stored in the device 10′ serves to feedthe helium-concentrating device 3′.

The gas stream 12′ containing a majority of nitrogen and a small amountof helium is purified by means of a purification means 13′ which removesthe impurities contained in the gas stream 12′ so as to produce arecycling gas stream 14′ sent to the compressor 2′ after having beenmixed with the initial gas stream 1′ to be treated.

When the hydrogen content is high, typically more than 4% by volume oreven 6%, the input of air into the hydrogen removal system 4′ in whichhydrogen and oxygen react is substantial. A large amount of nitrogen andargon is then introduced therein, which dimensions the PSA system 5′.

A purge used in the helium concentrator 3′ contains methane. It musttherefore be treated by means of a methane oxidation device to meet theenvironmental requirements.

It is necessary to have an air-separating unit (ASU) 11′ which producesliquid nitrogen to the specification compatible with the helium storages8′ (of the order of one ppm of methane).

SUMMARY

The inventors of the present invention thus developed a solution forsolving the problems raised above.

One subject of the present invention is a process for producing heliumfrom a source gas stream comprising at least helium, methane, nitrogenand hydrogen, comprising at least the following successive steps:

step a): introducing said source gas stream into at least onecompressor;

step b): removing hydrogen and methane by reaction of the streamobtained from step a) with oxygen;

step c): removing at least the impurities obtained from step b) bytemperature swing adsorption (TSA);

step d): partially condensing the stream obtained from step c) so as toproduce a liquid nitrogen stream and a gas stream predominantlycomprising helium;

step e): purifying the gas stream obtained from step d) so as toincrease the helium content by pressure swing adsorption (PSA) byremoving the nitrogen and the impurities contained in the gas streamobtained from step d).

According to other embodiments, a subject of the present invention is: Aprocess as defined previously, characterized in that the source gasstream comprises from 40% to 95% by volume of nitrogen, from 0.05% to40% by volume of helium, from 50 ppmv to 5% by volume of methane andfrom 1% to 10% by volume of hydrogen, preferably from 5% by volume to10% by volume of hydrogen.

A process as defined previously, characterized in that the source gasstream comprises from 40% to 60% by volume of nitrogen, from 30% to 50%by volume of helium, from 50 ppmv to 5% by volume of methane and from 1%to 10% by volume of hydrogen, preferably from 5% by volume to 10% byvolume of hydrogen.

A process as defined previously, comprising a step prior to step a) ofproducing the source gas stream to be treated by means of a nitrogenrejection unit or a natural gas liquefaction unit, said unit producing aliquid nitrogen stream used in step d) allowing partial condensation ofthe stream obtained from step c) so as to produce a liquid nitrogenstream and a gas stream predominantly comprising helium.

A process as defined previously, characterized in that the pressure onconclusion of step a) is between 15 bara and 35 bara, preferably between20 bara and 25 bara.

A process as defined previously, characterized in that the gas streamobtained from step b) comprises less than 1 ppm by volume of hydrogenand less than 1 ppm by volume of methane.

A process as defined previously, characterized in that said impuritiescontained in the gas stream obtained from step b) predominantly comprisecarbon dioxide and water.

A process as defined previously, characterized in that the liquidnitrogen stream obtained from step d) comprises more than 98.5% byvolume of nitrogen.

A process as defined previously, characterized in that said gas streamobtained from step d) comprises between 80% by volume and 95% by volumeof helium.

A process as defined previously, characterized in that said gas streamobtained from step e) comprises at least 99.9% by volume of helium.

A process as defined previously, characterized in that step b) consistsin placing the gas stream obtained from step a) in contact with oxygenand a catalytic bed comprising particles of at least one metal chosenfrom copper, platinum, palladium, osmium, iridium, ruthenium andrhodium, supported on a support that is chemically inert with respect tocarbon dioxide and water so as to react the methane and hydrogen withoxygen.

A process as defined previously, characterized in that it comprises anadditional step f) of liquefaction of the helium obtained from step e).

A process as defined previously, characterized in that the liquidnitrogen derived from step d) cools the helium liquefied in step f).

An installation for producing helium from a source gas mixturecomprising methane, helium, hydrogen and nitrogen, comprising at leastone compressor directly receiving the source gas mixture, at least onemeans for removing hydrogen and methane, at least one nitrogen-removingand helium-concentrating device, and at least one helium purificationmeans located downstream of the nitrogen-removing andhelium-concentrating device, characterized in that the means forremoving hydrogen and methane is located downstream of said at least onecompressor and upstream of the nitrogen-removing andhelium-concentrating device.

An installation as defined previously, characterized in that it alsocomprises a helium liquefaction device downstream of the heliumpurification means.

BRIEF DESCRIPTION OF THE DRAWINGS

For a further understanding of the nature and objects for the presentinvention, reference should be made to the following detaileddescription, taken in conjunction with the accompanying drawings, inwhich like elements are given the same or analogous reference numbersand wherein:

FIG. 1 illustrates a block flow diagram of a state of the art heliumpurification plant for separating helium from a nitrogen rejectionsystem for natural gas purification; and

FIG. 2 illustrates a block flow diagram of an embodiment of the inventedhelium purification plant for separating helium from a nitrogenrejection system for natural gas purification.

DESCRIPTION OF PREFERRED EMBODIMENTS

A source gas stream 1 containing at least helium, nitrogen, hydrogen andmethane is treated via a process that is the subject of the presentinvention so as to produce a pure helium stream, typically containingmore than 99.999% by volume of helium. The source stream 1 originates,for example, from a nitrogen rejection unit (NRU) 2 located downstreamof a cryogenic unit for treating natural gas.

The source stream 1 is introduced into a compressor 3 allowing the gasstream 4 to be compressed to a pressure of between 15 bara (barabsolute) and 35 bara, preferably between 20 bara and 25 bara. Thetemperature is the ambient temperature at the site of the installation.

The gas stream 4 is introduced into a unit 5 for removing hydrogen andmethane. This unit 5 consists, for example, of one or more reactors inseries containing a catalyst between grilles.

This catalyst is typically Pd/Al₂O₃. Catalytic oxidation between oxygenand the combustives (hydrogen/methane) takes place.

The hydrogen reacts with the oxygen to form water. Since this reactionis exothermic, the temperature rises.

To oxidize the methane also, higher temperatures are required. A highcontent of hydrogen at the inlet makes it possible to work at a hightemperature and to co-oxidize the methane (for example, with 2% ofhydrogen, the temperature rises to about 200° C., which is notsufficient to oxidize methane).

Thus, the hydrogen and methane contained in the initial source stream 1to be treated are oxidized with oxygen from the unit 5.

Impurities such as water and carbon dioxide are thus produced in the gasstream 6 leaving the unit 5. This gas stream 6 predominantly comprisesnitrogen and helium.

The exiting gas is cooled (against the ambient air or cooling water)before being sent to the adsorption unit 7. Some of the water thencondenses directly in a condensate recuperator. Some of the heatproduced may be recovered to be used in another process.

The gas stream 6 is then treated in an adsorption unit 7, such as atemperature swing adsorption (TSA) unit, so as to remove the water andcarbon dioxide from the gas stream 6. This results in a gas stream 8essentially comprising nitrogen and helium (i.e. comprising less than 5ppm by volume of methane, less than 1 ppm by volume of hydrogen, lessthan 0.1 ppm by volume of carbon dioxide and less than 0.1 ppm by volumeof water). The gas stream 8 is treated in a nitrogen-purifying andhelium-concentrating unit 9.

This unit 9 comprises at least one heat exchanger in which the gasstream is cooled from the ambient temperature (0° C.−40° C., forexample) to a temperature of between −180° C. and −195° C. On leavingthis heat exchanger, the gas stream is introduced, for example, into aphase-separating pot generating a liquid stream 10 and a gas stream 11.

The liquid stream 10 contains 98.8% by volume of nitrogen. This liquidstream 10 is sent to a liquid nitrogen storage device 12. It does notcontain any methane.

The gas stream 11 contains from 80% by volume to 95% by volume of heliumand from 5% by volume to 20% by volume of nitrogen. The stream 11 issent to a helium purification unit 13.

This purification unit 13 is, for example, a pressure swing adsorption(PSA) unit and produces two streams. One stream, 14, contains 99.9% byvolume of helium and another stream, 15, contains the rest of theelements (essentially nitrogen). The gas stream 15 is introduced into acompressor 16 and then mixed with the source gas stream 1 to be treated;this is a regeneration loop of the unit 13.

The helium-rich stream 14 may be sent to a helium liquefaction unit 17producing a liquid helium stream 18 conveyed to a storage device 19. Thepure liquid nitrogen 10 stored in the nitrogen storage device 12 may beused to maintain the temperature of the helium storage device 19.

According to a preferred embodiment, a liquid nitrogen stream 20produced by the nitrogen rejection unit 2 is introduced into thenitrogen-purifying and helium-concentrating unit 9. This makes itpossible to obtain the cooling power required and to thereby avoidinvestment in a dedicated air-separating unit, in contrast with theprocess illustrated in FIG. 1.

Use may also be made of another cold-generating fluid present on site(for example LNG) or of a high-pressure fluid which is expanded (viajoule Thomson expansion or turbines) to create the requiredrefrigeration.

Advantages of a process as illustrated in FIG. 2 that is the subject ofthe present invention relative to the process illustrated in FIG. 1 aredescribed below.

Simultaneous oxidation of hydrogen and methane takes place before heliumconcentration. The TSA 7 then functions under pressure, which ensuresbetter efficiency (reduction of the required volume of adsorbents andalso reduction of the heat consumption in the regeneration reheater).

The purge originating from the cryogenic helium-concentrating unit 9 nolonger contains any methane (which has been oxidized beforehand).

Methane-free liquid nitrogen 10 may thus be produced from the unit 9. Itsuffices to integrate this unit 9 with the helium-concentrating unit 2(NRU or natural gas liquefaction unit) to obtain the required coolingpower. This makes it possible to avoid investment in a dedicatedair-separating unit (ASU).

According to a particular mode of the invention, a stream 21 expandedbeforehand in the unit 9 containing nitrogen and helium is extractedfrom said unit 9 and then sent to a compressor 3 and/or 16. Thus, heliumobtained from the expansion of the liquid nitrogen from the unit 9 isrecycled so as to increase the percentage of helium produced.

For example, the stream 21 comprises between 40% and 50% by volume ofhelium and between 50% and 60% by volume of nitrogen.

The yield of the PSA unit 13 and its size are also greatly improved. Thehelium 11 is preconcentrated to about 90% in the PSA 13 (rather than 70%in the process of FIG. 1 and with a high content of hydrogen. The argonand oxygen impurities are also in a much lower amount (since the argonand oxygen condense out at the same time as the nitrogen).

There is also no more carbon dioxide or water to be treated in theentering gas. The pressure of the residual gas (offgas) of the PSA 13may also be reduced relative to that of the process illustrated in FIG.1 since they can return directly to the compressor 16 without passingbeforehand through a drying unit.

All these points make it possible to improve the yield of the PSA 13,which dimensions the return line and the compressor 3 of the stream 1 tobe treated (the energy consumption of the compressor is reduced).

The table below summarizes the compositions of the gas streams enteringthe helium purification unit (element numbered 13 in FIGS. 2 and 5′ inFIG. 1).

TABLE Composition of the gases entering the PSA Gas stream CompositionFIG. 1 FIG. 2 He mol % 69.48% 89.9697% N₂ mol % 29.94%  9.9979% CH₄ ppmv1 1 Ar ppmv 2658 181 H₂ ppmv <0.5 <0.5 Ne ppmv 300 300 CO ppmv 0 0 O₂ppmv 2703 143 H₂O saturated 0 CO₂ ppmv 355 <0.1 Total mol %   100%   100% Flow rate (sec) Nm³/h 4806 3713 Pressure bara 23.55 23.45Temperature ° C. 47 47

It will be understood that many additional changes in the details,materials, steps and arrangement of parts, which have been hereindescribed in order to explain the nature of the invention, may be madeby those skilled in the art within the principle and scope of theinvention as expressed in the appended claims. Thus, the presentinvention is not intended to be limited to the specific embodiments inthe examples given above.

1. A process for producing helium from a source gas stream (1)comprising at least helium, methane, nitrogen and hydrogen, the processcomprising at least the following successive steps: step a): introducingsaid source gas stream (1) into at least one compressor (3); step b):removing hydrogen and methane by reaction of the stream (4) obtainedfrom step a) with oxygen; step c): removing at least the impuritiesobtained from step b) by temperature swing adsorption (TSA); step d):partially condensing the stream (8) obtained from step c) so as toproduce a liquid nitrogen stream (10) and a gas stream (11)predominantly comprising helium; step e): purifying the gas stream (11)obtained from step d) so as to increase the helium content by pressureswing adsorption (PSA) by removing the nitrogen and the impuritiescontained in the gas stream (11) obtained from step d).
 2. The processof claim 1, wherein the source gas stream (1) comprises from 40% to 95%by volume of nitrogen, from 0.05% to 40% by volume of helium, from 50ppmv to 5% by volume of methane and from 1% to 10% by volume ofhydrogen.
 3. The process as claimed in claim 2, characterized in thatthe source gas stream (1) comprises from 40% to 60% by volume ofnitrogen, from 30% to 50% by volume of helium, from 50 ppmv to 5% byvolume of methane and from 1% to 10% by volume of hydrogen.
 4. Theprocess of claim 1 further comprising a step prior to step a) ofproducing the source gas stream (1) to be treated by means of a nitrogenrejection unit (2) or a natural gas liquefaction unit, said nitrogenrejection unit (2) or natural gas liquefaction unit producing a liquidnitrogen stream (20) used in step d) to partially condense the stream(8) obtained from step c).
 5. The process of claim 1, wherein thepressure of the source gas stream (1) on conclusion of step a) isbetween 15 bara and 35 bara.
 6. The process of claim 1, wherein the gasstream (6) obtained from step b) comprises less than 1 ppm by volume ofhydrogen and less than 1 ppm by volume of methane.
 7. The process ofclaim 1, wherein said impurities contained in the gas stream (6)obtained from step b) predominantly comprise carbon dioxide and water.8. The process of claim 1, wherein the liquid nitrogen stream obtainedfrom step d) comprises more than 98.5% by volume of nitrogen.
 9. Theprocess of claim 1, wherein said gas stream obtained from step d)comprises between 80% by volume and 95% by volume of helium.
 10. Theprocess of claim 1, wherein said gas stream obtained from step e)comprises at least 99.9% by volume of helium.
 11. The process of claim1, wherein in step b) the gas stream obtained from step a) is placed incontact with oxygen and a catalytic bed comprising particles of at leastone metal chosen from copper, platinum, palladium, osmium, iridium,ruthenium and rhodium, wherein the metal is supported on a support thatis chemically inert with respect to carbon dioxide and water, andwherein to the catalyst bed catalyzes a reaction of the methane andhydrogen with oxygen.
 12. The process of claim 1, further comprising anadditional step f) of liquefaction of the helium obtained from step e).13. The process of claim 1, wherein in the liquid nitrogen obtained fromstep d) cools the helium liquefied in step f).
 14. An installation forproducing helium from a source gas mixture (1) comprising methane,helium, hydrogen and nitrogen, comprising at least one compressor (3)directly receiving the source gas mixture (1), at least system (5) forremoving hydrogen and methane, at least one nitrogen-removing andhelium-concentrating device (9), and at least one helium purificationsystem (13) located downstream of the nitrogen-removing andhelium-concentrating device (9), wherein the system (5) for removinghydrogen and methane is located downstream of said at least onecompressor (3) and upstream of the nitrogen-removing andhelium-concentrating device (9).
 15. The installation of claim 14,further comprising a helium liquefaction device (17) downstream of thehelium purification means (13).